In flocculation treatment of water to be treated such as purified water, industrial water, and wastewater, for example, SS (suspended solids) in water to be treated is subjected to the flocculation treatment with an inorganic flocculant or an organic flocculant before solid-liquid separation such as sedimentation separation, pressure floatation separation, centrifugal separation, sand filtration, membrane separation. The flocculation state of the SS varies depending on pH, a flocculant dosing amount, stirring conditions, etc., and if the flocculation treatment is not performed under appropriate conditions, water quality deterioration may be caused in the water to be treated, and may affect the solid-liquid separation treatment at the next step.
For such flocculation treatment, a method of setting flocculation conditions in a laboratory test exists; however, if it takes time to set the flocculation conditions in actual flocculation treatment, the water quality of the water to be treated varies and the flocculation state of the SS cannot accurately be comprehended in some cases. Therefore, to set optimum flocculation conditions such as pH, a flocculant dosing amount, and stirring conditions, it is important to monitor a treatment state of the water to be treated during the flocculation treatment in real time and to monitor the flocculation state of the SS.
With respect to this flocculation monitoring, it is known that a laser light is applied to water to be treated to receive a scattered light from particles in the water to be treated and that after AM (amplitude modulation) detection is applied to the light reception signal, a minimum value of signal intensity is obtained so as to obtain a flocculant dosing amount from this minimum value (e.g., Patent Literature 1). In this flocculation monitoring, the minimum value of signal intensity of the scattered light is obtained to distinguish and detect a scattered light due to non-flocculated suspended solids from the scattered light due to flocculated matter in the water to be treated.
It is known that the laser light used for this flocculation monitoring is implemented by using a laser light emitted at predetermined time intervals by intermittently driving a laser diode (e.g., Patent Literature 2). An operating time of a laser-emitting element is extended by a light-emitting form shortening this light-emitting time.